After some thought I got answers of the back and the legs. My response was that they are both.
Technically they are transitional between the trunk and the lower extremities. Anatomically they are generally referred to as part of the trunk. Functionally, because of their actions, they are part of the lower extermities.
So just what do the gluteal muscles do? Their main actions are extension of the hip as in standing or walking, especially in climbing or stairs. They also help to perform lateral rotation of the hip (maximus), abductions and medial rotation of the hip (medius & minimus). Their major synergists (helpers) are the hamstrings and piriformis along with tensor fascia latae.
The attachements of these 3 muscles, maximus, medius and minimus are on the illium along the crest and then along the body of the illium moving inferiorly. The distal attachments are into the IT Band and the greater trochanter of the femur thus the hip movements which each of these muscles. The gluteals are stretched by their antagonists, theilipsoas, tensor fascia latae, the adductors and themselves in some cases.
So how does this affect your clients? Think of what they do for a living. How do they use their bodies? Or even what do they do occasionally that may affect their muscles? Of course anyone who walks or stands at all during the day will be using these muscles and/or overusing them. Conversely if they are not using them they could also be affected.
Low back pain may be associated with gluteals as well as any of the other muscles that attach to the illium at the iliac crest. Piriformis syndrome or sciatica may also be related to tight gluteal muscles since they sit superficial to piriformis. Skiers, runners as well as truck drivers and couch potatoes all may end up in your office with gluteal issues. Don't ever let them tell you that they don't use their glutes! Work them! They can be worked over the sheet without any embarrassment to your client!